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Basic Data Structures in Java
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Language Features
 Object Oriented
 Class – the basic entity of OOP
 Object – instance of a class
 Methods – process code within class
 Interpreted Lang – Bytecode
 Builtin support for threading (concurrent
execution different pieces of code)
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HelloWorld Again
public class HelloWorld {
public static void main(String arguments[]) {
System.out.println(“Hello World"); }
}
Note:
1. Any main ‘program’ must have a main method and
it must be declared static. No instance is required.
JVM can directly run this.
2. If a class has no main method, then the only way to
use it is to call from another class. In practice, many
classes are defined like this... See next page
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/**
* This is the template for a class
*/
public class test{
// instance variables private int x;
/** Constructor for objects of class test */
public test()
{// initialise instance variables
x = 10; }
/** An example of a method – */
public int sampleMethod(int y)
{// put your code here
return x + y;
}
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}
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Instances
We have a class test and a method sampleMethod
How do we use them from another program?
test test1; // declare a new ‘variable’
test1 of type test
test1 = new test();
int xyz1;
xyz1 = test1.sampleMethod(30);//we get 40
This is the essence of OO programming.
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OO features / conventions
 Class name starts with Upper case
 Variables start with lowercase, each word
with uppercase
 Method – lowercase
 Methods must return something else
declared as void
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Capitalization
Java is picky about capitalization;
System.out.println() vs.
system.out.println()
are not the same
HelloWorld and helloworld are not
the same!
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Datatypes – 8 of them
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boolean: true or false. 1 bit size
char: Unicode character. 16 bits.
byte: very small integer number; -128-127, 8 bits
short: a smallish integer number, -32,768 to 32,767
int: normal integer, 32 bits, -2,147,483,648 to
2,147,483,647
long: a really big 64 bit integer, -9223372036854775808
to + 9223372036854775807
float: 32 bit floating point number, -3.40292347E+38 to +
3.40292347E+38
double: 64 bit long floating point number,
good for -E+308 to +E+308 and about twice the
number of digits of precision
as a float
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Declaring Variables
 Variables are declared in the C style; the
type first, followed by the variables being
declared
InterCap Style
int
int
inputCount;
currentCount23, finalValue;
 Variable names must start with a letter or an
underscore character, “_”
 Cannot start with a number.
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3 types of variable declarations
instance, class and local variables
class Variables1 {
double salary = 2534.50 ;// instance variable
static int counter1; // class variable
public static void main (String args[]){
int temp1 = 0; // local variable defined inside a method
System.out.println
("counter1=" + counter1 + " temp1=" + temp1 );
} // end of method main
} // end of class Variables1
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Initialize
You can initialize variables in a declaration.
Double initialSpeed =10.0, finalSpeed=60.0,
currentSpeed = 0.0; char
endChar = ‘a’;
Note: Instance and static(=class) variables will be
initialized to default values.
Local variable will NOT be initialized automatically.
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Scope of a Variable
 IBM doc says “variable scope can be
confusing in Java”
 An instance method has access to all
(instance, class and local variables)
 A class method (= defined static) has access
to only class variables and local variables
 Local variables are limited to within a
method. Outside the method they do not
exist.
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Assignments
 Similar to any other language…
double currentSpeed = 0.0;
int
clock = 0;
currentSpeed = currentSpeed + 1.0;
clock++; // known as postincrement
// clock = clock + 1; // same as above
A number like 14.65 is treated as double!
float price1;
price1 = 14.65; Gives compile error
price1 = 14.65f; // ok now
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if .. else
if (booleanTest)
{ // code for Value is true. } else
{ // code for Value is false.}
Note!
eg:
int weight1 = 100, weight2=200;
if ( weight1 == weight2)
{System.out.print(“weight1 equals weight2 ");}
else
{System.out.print(“weight1 not equals weight2 ");}
 Note the difference between the equality operator (==) and
the assignment operator (=)
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String
Not a basic data type!
Used as if it is !
String in Java is implemented as a class
Note: not ‘string’, capital ‘S’ is required
String str1 = new String(“string value”);
String str2 = “ string value”;
Char char1 = ‘A’; //String and char are not the same!
More on String later
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Final Variables (=constants)
 A final variable is one that cannot be
changed. Any attempt to change it, will
result in a compile-time error. This is done
with the keyword ‘final’ in the declaration.
 Typically they are in CAPITAL letters
double final PI = 3.14159265359 ;
int final
SPEED_LIMIT = 70;
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Other Syntax
 while
 do ... while
 for ... loop
 Most are simple and straightforward
Switch – will be discussed later
All others see syntax page at the end ...
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Arrays
datatype arrayName [] = new datatype[25];
or datatype [] arrayName = new datatype[25];
eg:
String ErrorCode [] = new String[50];
The first/last elements are ErrorCode[0] ...
ErrorCode[49]
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Initializing Arrays
Method 1
String names[];
Names = new String[3];
Names[0] = “Ada”;
Names[1] = “Byron”;
Names[2] = “Napolean”;
Method 2
String names[] = {“Ada”; “Byron”; “Napolean”};
Here definition and initialize are done at once.
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Hello World Again
In the HelloWorld
public static void main (String args[]) { ...}
The main method takes an array as input
arguments. If I say
Java HelloWorld John David
These names are passed to args[0], args[1]...
Here follows the modified program
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Example
class Hello2 {
public static void main (String args[]) {
int i;
if (args.length == 0)
System.out.println("Hello Nobody ");
else
System.out.println("Arguments length= " + args.length);
for (i=0; i < args.length; i = i+1) {
System.out.println("Hello " + args[i]); }
}
}
Try Java Hello2 *
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What do you get? (c)http://www.geocities.com/idmssql
Javadoc – part of JSDK
Input is Java source
Output – html files with help information
C:\tvg\JCourse1>Javadoc TestGreeting.java
Loading source file TestGreeting.java...
Constructing Javadoc information...
Standard Doclet version 1.4.2_03
Generating constant-values.html...
Building tree for all the packages and classes...
Building index for all the packages and classes...
Generating overview-tree.html...
Generating index-all.html... Generating deprecated-list.html...
Building index for all classes...
Generating allclasses-frame.html...
Generating allclasses-noframe.html...
Generating index.html...Generating packages.html...
Generating TestGreeting.html...Generating package-list...
Generating help-doc.html...
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Java Buzzwords
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Java2 Platforms
- Standard Edition J2SE
- Enterprise Edition – J2EE
- Micro Edition – J2ME (PDA)
Every Platform has
 Run time – JRE (can be shipped with application)
 Dev Kit – JDK (includes JRE)
Enterprise Information System
 Servlet, JSP, J2EE Client, Applet...Application Server
 JDBC, Beans, JavaBeans, EJB ...
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